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WO3 toward an active phase during catalytic cycles of CO oxidation
Accepted Manuscript
A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic cycles of CO oxidation Byungwook Jeon , Ansoon Kim , Young-Ahn Lee , Hyungtak Seo , Yu Kwon Kim PII: DOI: Reference:
S0039-6028(17)30394-1 10.1016/j.susc.2017.08.007 SUSC 21071
To appear in:
Surface Science
Received date: Revised date: Accepted date:
26 May 2017 25 July 2017 5 August 2017
Please cite this article as: Byungwook Jeon , Ansoon Kim , Young-Ahn Lee , Hyungtak Seo , Yu Kwon Kim , A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic cycles of CO oxidation, Surface Science (2017), doi: 10.1016/j.susc.2017.08.007
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Highlights CO oxidation rate over Pd/WO3 depends strongly on the oxidation states of Pd and WO3. The most active phase is a mixture of Pd and PdO supported on WO3-x with W5+.
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The role of WO3 as a reversible oxygen storage is suggested.
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A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic
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cycles of CO oxidation Byungwook Jeon1, Ansoon Kim4, Young-Ahn Lee1, Hyungtak Seo1,3* and Yu Kwon Kim1,2* Department of Energy Systems Research, Ajou University, Suwon 16499, South Korea
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Department of Chemistry, Ajou University, Suwon 16499, South Korea
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Department of Materials Science and Engineering, Ajou University, Suwon 16499, South Korea
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Korea Research Institute of Standards and Science (KRISS) Gajeongro 267, Yuseong-gu,
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RECEIVED DATE (August 5, 2017)
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Daejeon 34113, South Korea
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CORRESPONDING AUTHOR FOOTNOTE *To whom correspondence should be addressed.
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Phone: 82-31-219-2896. Fax: 82-31-219-2969. Email: [email protected] (Y.K.K.)
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Phone: 82-31-219-3532. Fax: 82-31-219-1613. Email: [email protected] (H.S)
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ABSTRACT
CO oxidation over Pd/WO3 films prepared on a glass substrate has been examined at the substrate temperature of 150 – 250 °C and pressures less than 1 Torr with a stoichiometric
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mixture of CO and O2. Under the given reaction condition, the chemical states of the Pd/WO3 film gradually change into the most catalytically active form with the highest saturation reaction rate regardless of the initial oxidation states. The measured CO oxidation rate over the Pd/WO3 is
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strongly dependent on the chemical states of Pd and W. Either metallic Pd or fully oxidized PdO phase is not as catalytically active as the active form with mixed metallic Pd and thin PdO layers supported on WO3 with partially reduced W5+ state which is spontaneously obtained during the catalytic reaction cycles. Our results indicate that the facile oxygen transfer between Pd and WO3
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layers not only facilitate the spontaneous changes into the active form, but also act as a
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promotional role in CO oxidation over the Pd layer.
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KEYWORDS CO oxidation, XPS, Heterogeneous Catalysis, Pd/WO3, oxygen storage
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1. Introduction CO oxidation has been numerously studied over single crystal metal surfaces [1-3] as well as over oxide supported metal catalysts [4-6] since it has played a successful role in studying
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fundamental concepts in catalysis [7, 8], in addition to its relevance to catalytic reactions for the removal of automotive exhaust [9, 10].
Oxide-supported Pd catalysts [11] generally exhibit a high activity in CO oxidation and their activity can be further enhanced by a proper choice of support and preparation condition [12-15].
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They are known to be good catalysts for the three-way catalysts which convert the exhaust emission gases such as CO, NOx, and hydrocarbons to harmless CO2, N2, and H2O [16]. Among them, Pd/WO3 has been one of the promising candidates for the three-way catalyst [17]. In addition, Pd/WO3 system has been an attractive catalyst for number of studies including
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photocatalytic oxidation [18], selective isomerization [19], hydrogenation [20], carbon–carbon
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coupling reactions [21] and sensors for the detection of various gases including hydrogen [22], H2S [23] and NH3 [24]. Motivated by such diverse applications of Pd/WO3 and the importance of
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CO oxidation in the study of catalysts, we focus on the catalytic CO oxidation over Pd/WO 3 in
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Earlier studies suggest that CO oxidation over group 8 transition metal surfaces as well as those supported on oxide supports is generally facilitated by the presence of labile oxygen and
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weakly bound CO for subsequent formation of CO2 via Langmuir-Hinshelwood (LH) process [25-27] at low pressures. However, there are controversial understandings on the active phase of the Pd catalysts for CO oxidation. Some suggest that the presence of Pd oxides are important in catalytic activity [25, 26], but others report a poor CO oxidation activity of the oxide phase [28] and a high activity of chemisorbed oxygen on metallic Pd with low CO coverage [2]. Both
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chemisorbed oxygen and the surface oxide are also proposed to be active phases in the nearambient condition, too [29]. Recent theoretical calculations disclose candidates for the active oxide phases for CO oxidation such as Pd5O4[1] and PdO [25].
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The type of supports can have a strong influence on the chemical states of Pd and their catalytic activity in CO oxidation [13, 14]. The exact type and strength of metal-support interaction have a strong influence on the dispersion and oxidation state of Pd itself [15, 30-33]. Although the detailed dispersion and the actual oxidation state of Pd are assumed to play a
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significant role in the overall catalytic CO oxidation activity, the details are not so well understood unambiguously. Low catalytic activity over highly dispersed Pd on TiO2, Al2O3 and SiO2 has been attributed to the presence of PdO and lower dispersion of Pd is suggested for higher activity [13]. However, PdO phase is proposed to be the active phase on some systems
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such as PdO/CeO2 [34]. In addition, the PdO phase may change under reaction condition [35]
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and even transform into more complicated solid solution phases such as of Ce1−xPdxO2−δ due to even stronger support interactions [36].
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A recent study shows that Pd on WO3 tend to redisperse upon annealing up to 670 K suggesting a strong metal-support interaction [32]. Such a strong Pd-WO3 interaction may
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weaken a strong CO binding to Pd [37] and may play a beneficial role in enhancing CO
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oxidation rate at elevated temperatures (> 100 C). In addition, WO3 may act as a good oxygen storage via reversible oxidation-reduction of WO3 under the reaction condition [32, 33, 37, 38], as in the case of enhanced CO oxidation activity of Pd supported on oxides such as FeOx [31, 39] and CeOx [14, 15]. To have a better understanding on the active phase of Pd/WO3 catalysts and the roles of Pd and WO3 in catalytic CO oxidation, we studied reaction kinetics of catalytic CO oxidation over
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Pd/WO3. Any change in CO oxidation rate over Pd/WO3 film during the catalytic reaction cycles can be related with changes in the chemical states of the catalyst. Thus, by comparing the oxidation states of Pd and W after the cycles of catalytic reactions and the catalytic reaction rates,
and WO3 in the catalytic reaction cycles of CO oxidation.
2. Experimental Details
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we aim to understand the nature of the active phase of our Pd/WO3 catalysts and the roles of Pd
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Synthesis of Pd/WO3 A glass substrate of 4 x 2 cm2 were cleaned in a solution of ethanol and acetone with ultrasonication in for 10 min. A WO3 film was deposited by using radio frequency (RF) magnetron sputter (13.56 MHz, Sci & Tech, Korea) which was operated at a base pressure of about 5.5×10-6 Torr and a working pressure of 10 mTorr with Ar gas. The deposition time was
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50 min for the preparation of the WO3 film. Pd was evaporated on the WO3 film by using e-beam
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evaporator (Sci & Tech, Korea) which had a base pressure of 4.5×10-6 Torr. The deposition rate of Pd was monitored by quartz crystal microbalance (QCM) and was controlled to 0.1 Å/s.
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Characterization The morphology and structure of the Pd/WO3 films were examined by a field emission transmission electron microscope (FE-TEM, FEI Tecnai G2 F30 S-Twin). For the
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characterization of the surface chemical states of the Pd/WO3 films, X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI5000 Versa Probe II (Ulvac-PHI)
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using a monochromatic Al Kα source, which was maintained at a base pressure below 4×10-10 Torr. The samples were stored and handled in a quartz tube filled with N2 before being introduced into the XPS chamber to minimize any interaction with air.
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function was calibrated to give the binding energy (BE) of 83.96 eV for the Au 4f7/2 core level from a metallic gold film. The X-ray beam diameter was set to 200 μm and a charge neutralizer
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was used to minimize an undesirable charging effect. The binding energy for all spectra was calibrated by referring the C 1s peak from ubiquitous hydrocarbon contamination to 284.8 eV. The fitting analysis of the spectra was performed using a Shirley background subtraction and
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mixed Gaussian-Lorentzian components. CO oxidation reactivity measurements The CO oxidation reaction was studied in a vacuum tube equipped with a Baratron pressure gauge in the range of 10 Torr, which was pumped down to 1 x 10-4 Torr. The Pd/WO3 film grown on a glass substrate was placed in the vacuum tube
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which was wrapped with a heating jacket and K-type thermocouple. For systematic measurements on kinetics of CO oxidation reaction, CO oxidation reaction started directly after injecting reactants to reactor with catalysts at RT while the substrate temperature was maintained at 150 – 250 C. The change in the pressure was systematically monitored and the gas
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composition was monitored by a mass spectrometer in a UHV chamber which was connected to
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the reaction tube via a pinhole for a sampling of reactant gas. During the catalytic reaction cycles,
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only CO2 was detected as the reaction product.
3. Results and Discussion
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3.1 Structural Characterization of the Pd/WO3 film
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The morphology of the Pd/WO3 film grown on a glass substrate is shown in the SEM images of Figure 1.
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Figure 1. (a) The cross-sectional SEM image of the Pd/WO3 film grown on a glass substrate, (b) the cross-sectional TEM image of the Pd/WO3 film corresponding to the dashed square area of (a), and (c) the top-view SEM image of the Pd/WO3 film.
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The film thickness is measured to be about 610 nm (Figure 1(a)) and the top layer is viewed as
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a dry land with several 10-nm sized grains and many cracks in between. This provides the high surface area for the effective gas adsorption.
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The film shows a reproducible change in color when the surface is reduced with H2 or oxidized with O2 as the film is a good hydrogen sensor [22]. When the film is fully oxidized with O2 at 200 – 300 °C, the color changes to a reddish brown. As the film is exposed to a few Torr of pure H2, the color changes gradually to blue over several hours. The color further changes into a dark blue as the partial pressure of H2 increases further up to ~ 100 Torr.
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CO oxidation reaction rate on the Pd/WO3 film has been evaluated by monitoring the pressure change when the reactor is filled with a mixture of CO and O2 with a 2:1 ratio at 0.3 Torr. After the reaction, the mixture of gas is introduced into another chamber with a quadrupole mass
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spectrometer through a pinhole and the gas composition is analyzed. When the substrate temperature is lower than 400 K, no measurable change in the overall pressure is observed over hours suggesting no reaction occurs. Amount of the reactant gas adsorbed on the surface is not high enough to induce any meaningful change in the pressure. No change in the gas composition
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is observed from the mass spectrometer. As the substrate temperature is raised above 400 K, a gradual change in the overall pressure is observed as a result of stoichiometric oxidation reaction between CO and O2 over the substrate. After the completion of the CO oxidation, the pressure dropped to two-thirds (0.2 Torr) of the initial pressure (0.3 Torr) due to the mass balance of the
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stoichiometric CO oxidation. Also, the reaction product turns out to be CO2 only.
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3.2 Reaction kinetics of CO oxidation over the Pd/WO3 film
Figure 2. The extent of CO oxidation over Pd/WO3 in a fixed volume of reactant gas (CO: O2 = 2 : 1) at 0.3 Torr is monitored as a function of reaction time at the substrate temperature of 175 C (a). The results obtained at different substrate temperature (175 – 250 C) are also compared
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in a log scale in (b). The reaction rate is found to change depending on the extent of reaction as well as on the substrate temperature.
Figure 2 shows our results on temperature-dependent CO oxidation evaluated over the Pd/WO3
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at the substrate temperature of 175 – 250 C. The results obtained at 175 C (Figure 2(a)) show the characteristic features of CO oxidation over Pd/WO3 system. The reaction starts at a low rate at the early stage of reaction and increases slowly. As the reaction proceeds, the rate continues to increase slowly. As the extent of reaction exceeds about 80 %, there is a region where the rate is
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accelerated suddenly until the completion of the reaction. A significant enhancement in reaction rate is observed from oxygen-covered Pd surfaces under similar reaction conditions [40]. The depletion of CO in the gas phase may induce a favorable condition for the formation of oxygen
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covered surface for a higher rate.
Such variation in reaction rate is also observed at other temperatures in the temperature range
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of 175 – 250 C as shown in Figure 2(b). The reaction starts slowly and then proceeds into a steady rate. The steady rate is measured to be higher as the temperature increases. As the reaction
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approaches to completion, the rate shows a slight enhancement, which can be attributed in part to
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the decrease in CO content.
At even higher temperatures (>250 C), the reaction completes within a few tens of seconds
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and the rate is limited by the diffusion of reactant gas to the substrate [2]; at this temperature range, the measured reaction rate decreases as the reaction completes. Thus, we can assume that the diffusion rate of reactant in the gas phase does not limit the overall reaction at the temperature range of 175 – 250 C and the pressure of 0.3 Torr. Thus, the temperature and
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pressure range are selected for examining reaction kinetics on CO oxidation over the Pd/WO3
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system, such as activation energy barriers for CO oxidation.
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Figure 3. (a) The Arrhenius plot of the CO oxidation over Pd/WO3 at different stages of the reaction. (b) The calculated activation energy barrier is shown as a function of the extent of reaction.
Since the reaction rate changes over the extent of reaction, the change in the reaction rate is
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analyzed at each stage of the extent of reaction and is plotted against 1/T in Figure 3(a). In the plot, we find that the rate is higher with increasing the extent of reaction at each temperature
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until the completion of reaction. Also, at a fixed extent of reaction, the rate is higher as the
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temperature increases. At each stage of the reaction or the extent of reaction, we can determine the slope in the Arrhenius plot and it is clearly seen that the slopes are different with each other; it is observed to be high at the very early stage of reaction and decreases as the reaction approaches to a completion. It indicates that the apparent activation energy barrier is higher (lower) at the early (final) stage of the reaction; this explains the observation in Figure 2(a) that the rate is lower (higher) at the early (final) stage of the reaction. The change in the measured
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activation energy against the extent of reaction is shown in Figure 3(b). The activation energy barrier at the beginning is about 115 kJ/mol, but it rapidly approaches to a typical value of about 100 – 105 kJ/mol during the reaction (20 – 80 %). Then, it decreases to about ~ 80 kJ/mol as the
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reaction approaches to a completion. The turnover frequency at the reaction site may change as the reactant concentrations at the reaction site as well as in the gas phase change. In the beginning of reaction, a high coverage of CO on Pd surface is expected due to the high desorption temperature (~ 600 K) of CO on Pd;
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surface-bound CO may show a poisoning effect and reduce the reaction rate of CO oxidation over transition metals such as Pd [27] and Pt [5, 6, 41]. As the reaction proceeds and CO is consumed in the gas phase, the partial pressure of CO decreases. The corresponding decrease in the surface CO concentration may cause the gradual increase of reaction rate over the course of
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reaction.
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The oxidation state of Pd supported on the WO3 film may also change during the course of reaction. The measured energy barrier of 105 kJ/mol agrees well with those obtained for oxide
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supported small Pd particles [42, 43]. Higher energy barriers of 115 – 120 kJ/mol have been obtained from CO-inhibited low-temperature region over single crystal Pd surfaces–where
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reaction is limited by desorption of CO from Pd surface at < 580 K [42]. Thus, the low reaction rates with the high activation energy barriers observed at the beginning of reaction can be
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assigned for the reactivity of CO on a metallic Pd surface with high concentration of CO. As the reaction proceeds, surface oxygen population increases to form chemisorbed oxygen or a surface oxide layer (e.g., PdO) and the reaction rate is dominated by those involving those active oxygen species. As the reaction approaches to a completion, the gas phase CO concentration is reduced and the surface is more dominated by those on PdO. The activation energies for CO oxidation on
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a thin surface oxide (PdO) are suggested to be about 0.5 – 0.9 eV [25, 44]. Also, oxide supported Pd clusters can be readily oxidized into supported PdOx cluster [45] over which CO oxidation is proposed to occur via Mars-van Krevelen mechanism. Facile oxide formation can be is in part
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inferred from the similarity in CO oxidation between supported oxide clusters and the single
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crystal Pd surfaces [26, 29].
Figure 4. Cycles of CO oxidation reactions over Pd/WO3 in a fixed volume performed at PT = 0.3 Torr and Ts = 225 C. The net reaction rate increases with increasing cycles for both reduced and oxidized Pd/WO3, until a saturation rate is achieved at sufficiently high number of reaction cycles (> 8). The pictures of the Pd/WO3 film are also shown in the figure for the beginning and the end of the cycles.
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The relation between the catalytic CO oxidation activity and the initial oxidation states of Pd/WO3 is examined in Figure 4. The Pd/WO3 film is either reduced in H2 (P(H2) = 300 Torr) or oxidized in O2 (P(O2) = 300 Torr) at 300 C prior to the cycles of reactions. Then, the successive
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measurements of CO oxidation reaction rates are performed at the substrate temperature of 225 C with the mixture of CO and O2 (2:1 ratio) at P = 0.3 Torr. At the completion of CO oxidation in each reaction cycle, the reaction tube is pumped down below 10 -4 Torr while the substrate temperature is maintained to 225 C. Then, the mixture of fresh CO and O2 is introduced to the
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tube subsequently up to the same initial pressure (0.3 Torr) for the next cycle of reaction.
Figure 4 shows that the reaction rate is low for the first cycle for both cases; especially, the rate is much lower on the oxidized Pd/WO3 than on the reduced Pd/WO3; it takes about 1000 s for the completion of reaction on the reduced Pd/WO3 while it takes 4 times longer time (~ 4000 s) for
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the oxidized Pd/WO3. As the reaction cycle continues, we see that the rate becomes faster as the
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number of cycle increases until a saturation reaction rate is achieved after about 8 cycles; the reaction completes within 500 s when the reaction rate approaches to the maximum rate.
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Interestingly, the same saturation rate is achieved for both oxidized and reduced Pd/WO3 films, suggesting that the final chemical states of Pd/WO3 after the cycles of reactions would be the
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same regardless of the initial oxidation states.
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Figure 5. The change in the CO oxidation rate observed in Figure 4 is plotted with increasing number of cycles for both reduced (blue) and oxidized (red) Pd/WO3. The reaction rate approaches to a saturation value of 1.2 ×1015 CO2/s for both cases.
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The reaction rates calculated from the results in Figure 4 are plotted in Figure 5. Here, the rate is plotted against the cumulative total number of CO2 produced over cycles. At each cycle, the
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reaction rate changes over the extent of reaction. But, with increasing number of cycles, we find that the reaction rate increases for both reduced and oxidized Pd/WO3 films. The saturation rate
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is estimated to be about 1.2 × 1015 CO2/s for both cases. The observed variation in the reactivity
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strongly suggests that the surface chemical structure of the Pd/WO3 film evolves to an ‘active form’ with ‘active sites’ for CO oxidation reaction as the reaction proceeds. The active state is likely to be a partially oxidized form since both reduced and oxidized states of the Pd/WO3 film are not as active as the final ‘active state’ obtained after the long cycles of reaction in the CO and O2 mixture.
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The active form of Pd/WO3 is further annealed in vacuum at around 300 C and the reaction rate is also compared with those oxidized and reduced forms in Figure 5. The measured rate overlaps with that of the active states of Pd/WO3. The difference is that the rate is slightly
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enhanced at the end of reaction cycle as seen in Figure 2. The vacuum annealing would induce desorption of oxygen from the Pd layer which results in more metallic Pd exposed to the surface. In such a case, oxygen-covered Pd surface can be induced when CO depletes at the near end of
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the reaction for a condition for a hyperactive state for CO oxidation reaction.
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3.3 X-ray photoelectron study of the Pd/WO3 film
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Figure 6. Pd 3d core level spectra taken from Pd/WO3 at three different stages of as-synthesized, oxidized and reduced forms and those after CO oxidation.
To understand the changes in the oxidation state of Pd of our Pd/WO3 under different
preparation conditions of as-synthesized, oxidized and reduced states as well as those after CO oxidation, we show XPS spectra of Pd 3d core level taken from those samples in Figure 6. It shows that the major peak positions of Pd 3d core level for the as-synthesized and the reduced states are very similar to each other; the Pd 3d5/2 peak appears at 335.1 eV for both cases. On the
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other hand, the Pd peak for the oxidized Pd/WO3 is observed at 337 eV. Interestingly enough, the Pd core level spectral shapes of all three samples change after the CO oxidation in a way that they look very similar with each other. The major peaks for all the samples are positioned close
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to 335. 1 eV with a shoulder feature at 336.9 eV. Pd 3d5/2 peak has been observed at 335.0 – 335.2 eV for Pd/WO3 [46, 47]. Thus, the peak at 335.1 eV is attributed to thin metallic Pd films (Pd0) on WO3. Pd on WO3 prefers to be Pd2+ on WO3 [12, 48] and the binding energies of PdO are shifted by about 2 eV toward higher binding energies compared to that of metallic Pd. The Pd
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3d5/2 binding energies from PdO2 shift more to higher binding energies (up to about 340 eV) [49]. Thus, the observed peaks at 336.9 eV can be attributed to the oxidized state of Pd such as Pd2+ as in PdO. From this assignment, it is clear that the chemical states of Pd on the active form of Pd/WO3 is neither a metallic Pd, nor an oxidized PdO, but a mixture of Pd and PdO. The active
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form of mixtures (Pd and PdO) is reversibly formed during the cycles of CO oxidation reaction.
Figure 7. W 4f core level spectra taken from Pd/WO3 at three different stages of as-synthesized, oxidized and reduced forms and those after CO oxidation.
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Figure 7 shows W 4f core level spectra taken from the three samples before and after the CO oxidation. For the oxidized Pd/WO3, only a single W 4f7/2 peak at 35.8 eV is observed. This value is in the range of measured binding energies (35.3 – 35.8 eV) for W 4f7/2 peaks of WO3
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(W6+) [49-52]. As-synthesized Pd/WO3 displays the very similar spectral shape with the major peak at the same peak position as the oxidized one, suggesting that WO3 is the dominant phase in the as-synthesized one. But the W 4f7/2 peak has a low binding energy tail, suggesting the presence of additional component at lower binding energies. This is assigned to be W5+ [53].
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After reduction in H2, the W 4f7/2 peak displays a new distinct peak at even lower binding energy of 32 eV. This peak position is likely to be assigned to W2+ [32, 33, 38]. The shoulder feature at 34 eV is assigned to W4+ considering the earlier studies which assigned the W 4f7/2 peaks at 32.7 – 34.5 eV to W4+ [54-56].
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After the CO oxidation, we find again that the W 4f core level spectral shapes of all three
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samples change in a way that they look very similar with each other. The spectral shape shows a major peak at around 36.2 eV with a slope toward lower binding energies. The slope is fitted
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with a W 4f7/2 component at 35.2 eV, which can be associated with W5+ species. Thus, the fully oxidized WO3 in the Pd/WO3 system is not the most active phase. Also, the reduced state with
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mixed oxidation states of W4+ and W2+ is not active, either. Instead, the oxidized(reduced) states of WO3 in the Pd/WO3 system become reduced(oxidized) reversibly during the catalytic cycles
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of CO oxidation into the ‘active state’ of WO3 which is composed of a major component of W6+ with a contribution of W5+ state. This result shows the capability of WO3 to absorb(release) oxygen into(from) the lattice at the temperature of only 225 C during the catalytic cycles of CO oxidation reaction.
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Figure 8. O 1s core level spectra taken from Pd/WO3 at three different stages of as-synthesized, oxidized and reduced forms and those after CO oxidation. O 1s overlaps with Pd 3p3/2.
The corresponding O 1s core level spectra are explored in Figure 8. The O 1s features taken
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from the three different samples indicate that the major peak associated with WO3 appears at
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532.2 eV. This can be associated with surface OH species [57], but also the feature can overlap with the Pd 3p3/2 feature which may dominate at this position. The bulk component of O 1s from
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WO3 is not resolved and appear as a slope in the low binding energy side of the major peak for the as-synthesized and reduced cases. But, it appears as a distinct peak at 530.8 eV for the
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oxidized one. Since all the three samples are exposed to air before XPS analysis, all the features that are associated with the oxygenate species (e.g., H2O) weakly adsorbed on the surface may
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also be present at the surface and constitute the slope toward higher binding energies [58]. The O 1s contribution from PdO is not resolved in this case due to the overlap of the O 1s components from both oxides at around 530 – 531 eV. After the CO oxidation reaction, the bulk component of WO3 is relatively well distinguished from the contribution of Pd 3p3/2 as well as that of surface OH. Probably, the handling of the samples in inert N2 atmosphere after the CO oxidation has
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limited surface contamination by moisture and has left the features originating from the in-situ condition clearly resolved. Our XPS results show that the transformation of the reduced (oxidized) form of Pd/WO3 into
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the catalytically active form involves the incorporation (release) of the reactant oxygen (lattice oxygen) into the lattice (the gas phase as CO2). The oxidation states of Pd (Pd2+) and W (W6+) in the oxidized form decrease to Pd0 and W5+ simultaneously during the CO oxidation cycles. During this process, the lattice oxygen atoms in the PdO and WO3 need to be released into the
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gas phase as CO2 via CO oxidation. Here, the lattice oxygen atoms in the PdO layer can be directly consumed by the reaction with CO to form CO2. However, the oxygen in WO3 needs to be transferred to the Pd layer for the reaction with CO to form CO2 which is subsequently released into the gas phase. Similarly, the oxidation of the reduced form (Pd/WOx) with Pd0 and
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W4+ (as well as W2+) species into the active form occurs via a simultaneous increase of the
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oxidation states of both Pd and W, which requires the consumption of oxygen from the reactant gas. The oxidation of the reduced W species may occur via a direct reaction of WOx with O2 or a
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transfer of oxygen from the Pd layer into the WOx. The above two processes of transformation are reverse to each other in terms of the oxygen flow in the matrix of Pd/WO3. The oxidation of
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the reduced form is the consumption of the reactant O2 into the lattices of Pd/WOx and the reduction of the oxidized form is the release of the lattice oxygen atoms into the gas phase as
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CO2.
During the gradual change in the oxidation states of the Pd/WO3 film into the active state, the
consumption (release) of oxygen from the stoichiometric mixture of CO and O2 would cause an imbalance in the stoichiometric consumption of all reactant molecules at the end of each cycle. The amount of O2 that may be consumed (released) from (into) the gas phase from the reduced
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(oxidized) Pd/WO3 film is calculated from the size of the sample and the thickness of the Pd layer and is estimated to be about 2 x 1016. Considering the total O2 molecules involved in the reaction cycles (~ 1018), it is likely that less than 1 % of O2 is used to induce the gradual changes
stoichiometric reaction between CO and O2 at each cycle.
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in the oxidation states at each cycle. This fact makes us to assume a near completion of
The gradual changes in the oxidation states of the reduced and oxidized forms continue until the active form is achieved. The catalytic cycle of CO oxidation occurs without any further
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change in the oxidation states of Pd and W at the active state with the highest CO oxidation rates. Thus, the active form of Pd/WO3 can be considered to be the result of an equilibrium between the two processes of oxidation and reduction within the matrix of Pd/WO3. This observation can be only successfully explained under the assumption of (1) the facile exchanges of oxygen
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between as well as within the lattices of Pd and WO3 and (2) the promotional role of the oxygen
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transfer between Pd and WO3 in the catalytic CO oxidation rate over the Pd/WO3 catalyst. The promotional role of the oxygen exchange of oxygen between Pd and WO3 can be further
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evidenced from the observation of the oxidation states of Pd and W in the active form. Figure 6 reveals that Pd in the active phase consists of both metallic Pd and PdO. Also, the results in
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Figure 7 show that the tungsten oxide support layer prefers the mixed states of W5+ and W6+ state (as in WO3) in its active form over which the highest catalytic reaction rates are observed. The
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transfer of oxygen between and within the matrix of Pd and WO3 is likely to be enhanced when the catalyst is in the mixed oxidation states as observed in this study. The observation of the mixed oxidation states in its active form is consistent with the early study suggesting that the reversible formation of reduced W5+ state promotes Pd to act as an ‘oxygen scavenger’ [37]. The reduced W (W5+) may react with O2 in the gas phase or capture oxygen from PdO to become W6+.
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Then, the oxidized WO3 may lose oxygen to Pd to form the W5+ species. The facile transfer of oxygen from WO3 to Pd can promote CO oxidation by providing a labile oxygen on Pd. However, an even more direct evidence on the role of WO3 as a reversible oxygen storage in CO
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oxidation may be obtained from an in-situ observation of any changes in the chemical states under reaction condition.
4. Conclusions
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In conclusion, we have measured the CO oxidation rate over Pd/WO3 films prepared on a glass substrate at the substrate temperature of 150 – 250 °C and a pressure less than 1 Torr. Under the exposure of reactant gas mixture with the stoichiometric CO/O2 ratio of 2:1, we find that the kinetic rate follows Arrhenius behavior with activation energies predicted for CO oxidation over
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the Pd layer. In addition, the catalytic CO oxidation kinetics is strongly dependent on the
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chemical states of the Pd/WO3 catalyst. Both the oxidized and the reduced forms are not as active as the active form which has partially oxidized Pd layers supported on WO3-x. The active
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form is spontaneously obtained during the cycles of CO oxidation by a gradual change in the oxidation states of Pd and W from the oxidized and reduced states into the mixed oxidation states
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of Pd (Pd0 and Pd2+) and W (W5+ and W6+). The observation of the facile oxygen flow within the matrix during CO oxidation especially in its active form is consisted with the promotional role of
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the WO3 support as a reversible oxygen storage in the catalytic CO oxidation over the Pd layer. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
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Technology (NRF-2015R1D1A1A02062151 & NRF-2016R1D1A1B03931639) and the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, industry & Energy (No.
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20154010200820).
References
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AN US
[1] Z. Duan, G. Henkelman, ACS Catal., 4 (2014) 3435-3443. [2] F. Gao, Y. Wang, Y. Cai, D.W. Goodman, J. Phys. Chem. C, 113 (2009) 174-181. [3] F. Gao, Y. Cai, K.K. Gath, Y. Wang, M.S. Chen, Q.L. Guo, D.W. Goodman, J. Phys. Chem. C, 113 (2009) 182-192. [4] S. Salomons, R.E. Hayes, M. Votsmeier, A. Drochner, H. Vogel, S. Malmberg, J. Gieshoff, Appl. Catal. B, 70 (2007) 305-313. [5] E.M.C. Alayon, J. Singh, M. Nachtegaal, M. Harfouche, J.A. van Bokhoven, J. Catal., 263 (2009) 228-238. [6] J. Singh, E. Alayon, M. Tromp, O.V. Safonova, P. Glatzel, M. Nachtegaal, R. Frahm, J.A. van Bokhoven, Ang. Chem., 47 (2008) 9260-9264. [7] R. Imbihl, G. Ertl, Chem. Rev., 95 (1995) 697–733. [8] M. Valden, X. Lai, D.W. Goodman, science, 281 (1998) 1647-1650. [9] H. Gandhi, G. Graham, R.W. McCabe, J. Catal., 216 (2003) 433-442. [10] R.M. Heck, R.J. Farrauto, Appl. Catal. A, 221 (2001) 443-457. [11] Y. Zhou, Z. Wang, C. Liu, Catal. Sci. Technol., 5 (2015) 69-81. [12] K. Okumura, M. Niwa, Catal. Surv. Jpn., 5 (2002) 121-126. [13] S.N. Pavlova, V.A. Sadykov, V.A. Razdobarov, E.A. Paukshtis, J. Catal., 161 (1996) 507516. [14] A. Satsuma, K. Osaki, M. Yanagihara, J. Ohyama, K. Shimizu, Appl. Catal. B, 132–133 (2013) 511-518. [15] E.M. Slavinskaya, Y.T. Kardash, O.A. Stonkus, R.V. Gulyaev, I.N. Lapin, V.A. Svetlichnyi, A.I. Boronin, Catal. Sci. Technol., 6 (2016) 6650-6666. [16] J. Wang, H. Chen, Z. Hu, M. Yao, Y. Li, Catal. Rev., 57 (2015) 79-144. [17] K.M. Adams, H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) 207-212. [18] Y. Sakai, A. Shimanaka, M. Shioi, S. Kato, S. Satokawa, T. Kojima, A. Yamasaki, Catal. Today, 241, Part A (2015) 2-7. [19] C. Bigey, G. Maire, J. Catal., 196 (2000) 224-240. [20] M.A. Kulagina, E.Y. Gerasimov, T.Y. Kardash, P.A. Simonov, A.V. Romanenko, Catal. Today, 246 (2015) 72-80. [21] Z. Lou, Q. Gu, Y. Liao, S. Yu, C. Xue, Appl. Catal. B, 184 (2016) 258-263. [22] Y.-A. Lee, S.S. Kalanur, G. Shim, J. Park, H. Seo, Sens. Actuators, B, 238 (2017) 111-119. [23] N.-H. Kim, S.-J. Choi, D.-J. Yang, J. Bae, J. Park, I.-D. Kim, Sens. Actuators, B, 193 (2014) 574-581.
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
CR IP T
[24] P. Van Tong, N.D. Hoa, N. Van Duy, D.T.T. Le, N. Van Hieu, Sens. Actuators, B, 223 (2016) 453-460. [25] J.T. Hirvi, T.-J.J. Kinnunen, M. Suvanto, T.A. Pakkanen, J.K. Nørskov, J. Chem. Phys., 133 (2010) 084704. [26] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci., 552 (2004) 229-242. [27] S.M. McClure, D.W. Goodman, Chem. Phys. Lett., 469 (2009) 1-13. [28] G. Zheng, E.I. Altman, J. Phys. Chem. B, 106 (2002) 1048–1057. [29] S. Blomberg, M.J. Hoffmann, J. Gustafson, N.M. Martin, V.R. Fernandes, A. Borg, Z. Liu, R. Chang, S. Matera, K. Reuter, E. Lundgren, Phys. Rev. Lett., 110 (2013) 117601. [30] A.S. Ivanova, E.M. Slavinskaya, R.V. Gulyaev, V.I. Zaikovskii, О.А. Stonkus, I.G. Danilova, L.M. Plyasova, I.A. Polukhina, A.I. Boronin, Appl. Catal. B, 97 (2010) 57-71. [31] T. Schalow, M. Laurin, B. Brandt, S. Schauermann, S. Guimond, H. Kuhlenbeck, D.E. Starr, S.K. Shaikhutdinov, J. Libuda, H.J. Freund, Ang. Chem., 44 (2005) 7601-7605. [32] I. Jirka, J. Plšek, F. Šutara, V. Matolín, V. Cháb, K.C. Prince, J. Phys. Chem. B, 110 (2006) 23837-23844. [33] J. Plšek, I. Jirka, V. Nikolajenko, Z. Knor, Surf. Sci., 600 (2006) 3943-3949. [34] A. Hadi, I.I. Yaacob, Catal. Today, 96 (2004) 165-170. [35] S.-H. Oh, G.B. Hoflund, J. Phys. Chem. A, 110 (2006) 7609-7613. [36] L. Meng, A.-P. Jia, J.-Q. Lu, L.-F. Luo, W.-X. Huang, M.-F. Luo, J. Phys. Chem. C, 115 (2011) 19789-19796. [37] A.M. Sica, C.E. Gigola, Appl. Catal. A, 239 (2003) 121-139. [38] K. Mašek, S. Nemšák, M. Mravcáková, P. Blumentrit, T. Skála, M. Škoda, V. Matolín, J. Phys. Conf. Ser., 100 (2008) 012008. [39] L. Liu, F. Zhou, L. Wang, X. Qi, F. Shi, Y. Deng, J. Catal., 274 (2010) 1-10. [40] M.S. Chen, Y. Cai, Z. Yan, K.K. Gath, S. Axnanda, D.W. Goodman, Surf. Sci., 601 (2007) 5326-5331. [41] A.D. Allian, K. Takanabe, K.L. Fujdala, X. Hao, T.J. Truex, J. Cai, C. Buda, M. Neurock, E. Iglesia, J. Am. Chem. Soc., 133 (2011) 4498-4517. [42] D.W. Goodman, Chem. Rev., 95 (1995) 523-536. [43] L. Kieken, M. Boudart, Catal. Lett., 17 (1993) 1-10. [44] J. Rogal, K. Reuter, M. Scheffler, Phys. Rev. Lett., 98 (2007) 046101. [45] B. Huber, P. Koskinen, H. Hakkinen, M. Moseler, Nat. Mater., 5 (2006) 44-47. [46] S. Kim, S. Park, S. Park, C. Lee, Sens. Actuators, B, 209 (2015) 180-185. [47] M. Ranjbar, N. Tahmasebi Garavand, S.M. Mahdavi, A. Iraji zad, Sol. Energy Mater. Sol. Cells, 94 (2010) 201-206. [48] K. Okumura, T. Kobayashi, H. Tanaka, M. Niwa, Appl. Catal. B, 44 (2003) 325-331. [49] A. Boudiba, P. Roussel, C. Zhang, M.-G. Olivier, R. Snyders, M. Debliquy, Sens. Actuators, B, 187 (2013) 84-93. [50] P. Charton, L. Gengembre, P. Armand, J. Solid State Chem., 168 (2002) 175-183. [51] J.N. Fiedor, A. Proctor, M. Houalla, D.M. Hercules, Surf. Inter. Anal., 23 (1995) 204-212. [52] A. Boudiba, C. Zhang, P. Umek, C. Bittencourt, R. Snyders, M.-G. Olivier, M. Debliquy, Int. J. Hydrogen Energy, 38 (2013) 2565-2577. [53] K. Mašek, J. Libra, T. Skála, M. Cabala, V. Matolín, V. Cháb, K.C. Prince, Surf. Sci., 600 (2006) 1624-1627. [54] W. Grünert, E.S. Shpiro, R. Feldhaus, K. Anders, G.V. Antoshin, K.M. Minachev, J. Catal., 107 (1987) 522-534.
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AC
CE
PT
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M
AN US
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[55] P. Biloen, G.T. Pott, J. Catal., 30 (1973) 169-174. [56] C. Bigey, L. Hilaire, G. Maire, J. Catal., 184 (1999) 406-420. [57] J. Li, Y. Liu, Z. Zhu, G. Zhang, T. Zou, Z. Zou, S. Zhang, D. Zeng, C. Xie, Sci. Rep., 3 (2013) 2409. [58] J.-C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys., 2 (2000) 1319-1324.
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Graphical Abstract
A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic cycles of CO oxidation Byungwook Jeon , Ansoon Kim , Young-Ahn Lee , Hyungtak Seo , Yu Kwon Kim PII: DOI: Reference:
S0039-6028(17)30394-1 10.1016/j.susc.2017.08.007 SUSC 21071
To appear in:
Surface Science
Received date: Revised date: Accepted date:
26 May 2017 25 July 2017 5 August 2017
Please cite this article as: Byungwook Jeon , Ansoon Kim , Young-Ahn Lee , Hyungtak Seo , Yu Kwon Kim , A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic cycles of CO oxidation, Surface Science (2017), doi: 10.1016/j.susc.2017.08.007
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Highlights CO oxidation rate over Pd/WO3 depends strongly on the oxidation states of Pd and WO3. The most active phase is a mixture of Pd and PdO supported on WO3-x with W5+.
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The role of WO3 as a reversible oxygen storage is suggested.
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A spontaneous change in the oxidation states of Pd/WO3 toward an active phase during catalytic
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cycles of CO oxidation Byungwook Jeon1, Ansoon Kim4, Young-Ahn Lee1, Hyungtak Seo1,3* and Yu Kwon Kim1,2* Department of Energy Systems Research, Ajou University, Suwon 16499, South Korea
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Department of Chemistry, Ajou University, Suwon 16499, South Korea
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Department of Materials Science and Engineering, Ajou University, Suwon 16499, South Korea
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Korea Research Institute of Standards and Science (KRISS) Gajeongro 267, Yuseong-gu,
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RECEIVED DATE (August 5, 2017)
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Daejeon 34113, South Korea
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1
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CORRESPONDING AUTHOR FOOTNOTE *To whom correspondence should be addressed.
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Phone: 82-31-219-2896. Fax: 82-31-219-2969. Email: [email protected] (Y.K.K.)
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Phone: 82-31-219-3532. Fax: 82-31-219-1613. Email: [email protected] (H.S)
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ABSTRACT
CO oxidation over Pd/WO3 films prepared on a glass substrate has been examined at the substrate temperature of 150 – 250 °C and pressures less than 1 Torr with a stoichiometric
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mixture of CO and O2. Under the given reaction condition, the chemical states of the Pd/WO3 film gradually change into the most catalytically active form with the highest saturation reaction rate regardless of the initial oxidation states. The measured CO oxidation rate over the Pd/WO3 is
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strongly dependent on the chemical states of Pd and W. Either metallic Pd or fully oxidized PdO phase is not as catalytically active as the active form with mixed metallic Pd and thin PdO layers supported on WO3 with partially reduced W5+ state which is spontaneously obtained during the catalytic reaction cycles. Our results indicate that the facile oxygen transfer between Pd and WO3
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layers not only facilitate the spontaneous changes into the active form, but also act as a
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promotional role in CO oxidation over the Pd layer.
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KEYWORDS CO oxidation, XPS, Heterogeneous Catalysis, Pd/WO3, oxygen storage
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1. Introduction CO oxidation has been numerously studied over single crystal metal surfaces [1-3] as well as over oxide supported metal catalysts [4-6] since it has played a successful role in studying
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fundamental concepts in catalysis [7, 8], in addition to its relevance to catalytic reactions for the removal of automotive exhaust [9, 10].
Oxide-supported Pd catalysts [11] generally exhibit a high activity in CO oxidation and their activity can be further enhanced by a proper choice of support and preparation condition [12-15].
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They are known to be good catalysts for the three-way catalysts which convert the exhaust emission gases such as CO, NOx, and hydrocarbons to harmless CO2, N2, and H2O [16]. Among them, Pd/WO3 has been one of the promising candidates for the three-way catalyst [17]. In addition, Pd/WO3 system has been an attractive catalyst for number of studies including
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photocatalytic oxidation [18], selective isomerization [19], hydrogenation [20], carbon–carbon
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coupling reactions [21] and sensors for the detection of various gases including hydrogen [22], H2S [23] and NH3 [24]. Motivated by such diverse applications of Pd/WO3 and the importance of
this study.
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CO oxidation in the study of catalysts, we focus on the catalytic CO oxidation over Pd/WO 3 in
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Earlier studies suggest that CO oxidation over group 8 transition metal surfaces as well as those supported on oxide supports is generally facilitated by the presence of labile oxygen and
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weakly bound CO for subsequent formation of CO2 via Langmuir-Hinshelwood (LH) process [25-27] at low pressures. However, there are controversial understandings on the active phase of the Pd catalysts for CO oxidation. Some suggest that the presence of Pd oxides are important in catalytic activity [25, 26], but others report a poor CO oxidation activity of the oxide phase [28] and a high activity of chemisorbed oxygen on metallic Pd with low CO coverage [2]. Both
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chemisorbed oxygen and the surface oxide are also proposed to be active phases in the nearambient condition, too [29]. Recent theoretical calculations disclose candidates for the active oxide phases for CO oxidation such as Pd5O4[1] and PdO [25].
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The type of supports can have a strong influence on the chemical states of Pd and their catalytic activity in CO oxidation [13, 14]. The exact type and strength of metal-support interaction have a strong influence on the dispersion and oxidation state of Pd itself [15, 30-33]. Although the detailed dispersion and the actual oxidation state of Pd are assumed to play a
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significant role in the overall catalytic CO oxidation activity, the details are not so well understood unambiguously. Low catalytic activity over highly dispersed Pd on TiO2, Al2O3 and SiO2 has been attributed to the presence of PdO and lower dispersion of Pd is suggested for higher activity [13]. However, PdO phase is proposed to be the active phase on some systems
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such as PdO/CeO2 [34]. In addition, the PdO phase may change under reaction condition [35]
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and even transform into more complicated solid solution phases such as of Ce1−xPdxO2−δ due to even stronger support interactions [36].
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A recent study shows that Pd on WO3 tend to redisperse upon annealing up to 670 K suggesting a strong metal-support interaction [32]. Such a strong Pd-WO3 interaction may
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weaken a strong CO binding to Pd [37] and may play a beneficial role in enhancing CO
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oxidation rate at elevated temperatures (> 100 C). In addition, WO3 may act as a good oxygen storage via reversible oxidation-reduction of WO3 under the reaction condition [32, 33, 37, 38], as in the case of enhanced CO oxidation activity of Pd supported on oxides such as FeOx [31, 39] and CeOx [14, 15]. To have a better understanding on the active phase of Pd/WO3 catalysts and the roles of Pd and WO3 in catalytic CO oxidation, we studied reaction kinetics of catalytic CO oxidation over
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Pd/WO3. Any change in CO oxidation rate over Pd/WO3 film during the catalytic reaction cycles can be related with changes in the chemical states of the catalyst. Thus, by comparing the oxidation states of Pd and W after the cycles of catalytic reactions and the catalytic reaction rates,
and WO3 in the catalytic reaction cycles of CO oxidation.
2. Experimental Details
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we aim to understand the nature of the active phase of our Pd/WO3 catalysts and the roles of Pd
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Synthesis of Pd/WO3 A glass substrate of 4 x 2 cm2 were cleaned in a solution of ethanol and acetone with ultrasonication in for 10 min. A WO3 film was deposited by using radio frequency (RF) magnetron sputter (13.56 MHz, Sci & Tech, Korea) which was operated at a base pressure of about 5.5×10-6 Torr and a working pressure of 10 mTorr with Ar gas. The deposition time was
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50 min for the preparation of the WO3 film. Pd was evaporated on the WO3 film by using e-beam
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evaporator (Sci & Tech, Korea) which had a base pressure of 4.5×10-6 Torr. The deposition rate of Pd was monitored by quartz crystal microbalance (QCM) and was controlled to 0.1 Å/s.
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Characterization The morphology and structure of the Pd/WO3 films were examined by a field emission transmission electron microscope (FE-TEM, FEI Tecnai G2 F30 S-Twin). For the
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characterization of the surface chemical states of the Pd/WO3 films, X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI5000 Versa Probe II (Ulvac-PHI)
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using a monochromatic Al Kα source, which was maintained at a base pressure below 4×10-10 Torr. The samples were stored and handled in a quartz tube filled with N2 before being introduced into the XPS chamber to minimize any interaction with air.
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function was calibrated to give the binding energy (BE) of 83.96 eV for the Au 4f7/2 core level from a metallic gold film. The X-ray beam diameter was set to 200 μm and a charge neutralizer
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was used to minimize an undesirable charging effect. The binding energy for all spectra was calibrated by referring the C 1s peak from ubiquitous hydrocarbon contamination to 284.8 eV. The fitting analysis of the spectra was performed using a Shirley background subtraction and
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mixed Gaussian-Lorentzian components. CO oxidation reactivity measurements The CO oxidation reaction was studied in a vacuum tube equipped with a Baratron pressure gauge in the range of 10 Torr, which was pumped down to 1 x 10-4 Torr. The Pd/WO3 film grown on a glass substrate was placed in the vacuum tube
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which was wrapped with a heating jacket and K-type thermocouple. For systematic measurements on kinetics of CO oxidation reaction, CO oxidation reaction started directly after injecting reactants to reactor with catalysts at RT while the substrate temperature was maintained at 150 – 250 C. The change in the pressure was systematically monitored and the gas
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composition was monitored by a mass spectrometer in a UHV chamber which was connected to
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the reaction tube via a pinhole for a sampling of reactant gas. During the catalytic reaction cycles,
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only CO2 was detected as the reaction product.
3. Results and Discussion
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3.1 Structural Characterization of the Pd/WO3 film
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The morphology of the Pd/WO3 film grown on a glass substrate is shown in the SEM images of Figure 1.
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Figure 1. (a) The cross-sectional SEM image of the Pd/WO3 film grown on a glass substrate, (b) the cross-sectional TEM image of the Pd/WO3 film corresponding to the dashed square area of (a), and (c) the top-view SEM image of the Pd/WO3 film.
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The film thickness is measured to be about 610 nm (Figure 1(a)) and the top layer is viewed as
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a dry land with several 10-nm sized grains and many cracks in between. This provides the high surface area for the effective gas adsorption.
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The film shows a reproducible change in color when the surface is reduced with H2 or oxidized with O2 as the film is a good hydrogen sensor [22]. When the film is fully oxidized with O2 at 200 – 300 °C, the color changes to a reddish brown. As the film is exposed to a few Torr of pure H2, the color changes gradually to blue over several hours. The color further changes into a dark blue as the partial pressure of H2 increases further up to ~ 100 Torr.
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CO oxidation reaction rate on the Pd/WO3 film has been evaluated by monitoring the pressure change when the reactor is filled with a mixture of CO and O2 with a 2:1 ratio at 0.3 Torr. After the reaction, the mixture of gas is introduced into another chamber with a quadrupole mass
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spectrometer through a pinhole and the gas composition is analyzed. When the substrate temperature is lower than 400 K, no measurable change in the overall pressure is observed over hours suggesting no reaction occurs. Amount of the reactant gas adsorbed on the surface is not high enough to induce any meaningful change in the pressure. No change in the gas composition
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is observed from the mass spectrometer. As the substrate temperature is raised above 400 K, a gradual change in the overall pressure is observed as a result of stoichiometric oxidation reaction between CO and O2 over the substrate. After the completion of the CO oxidation, the pressure dropped to two-thirds (0.2 Torr) of the initial pressure (0.3 Torr) due to the mass balance of the
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stoichiometric CO oxidation. Also, the reaction product turns out to be CO2 only.
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3.2 Reaction kinetics of CO oxidation over the Pd/WO3 film
Figure 2. The extent of CO oxidation over Pd/WO3 in a fixed volume of reactant gas (CO: O2 = 2 : 1) at 0.3 Torr is monitored as a function of reaction time at the substrate temperature of 175 C (a). The results obtained at different substrate temperature (175 – 250 C) are also compared
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in a log scale in (b). The reaction rate is found to change depending on the extent of reaction as well as on the substrate temperature.
Figure 2 shows our results on temperature-dependent CO oxidation evaluated over the Pd/WO3
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at the substrate temperature of 175 – 250 C. The results obtained at 175 C (Figure 2(a)) show the characteristic features of CO oxidation over Pd/WO3 system. The reaction starts at a low rate at the early stage of reaction and increases slowly. As the reaction proceeds, the rate continues to increase slowly. As the extent of reaction exceeds about 80 %, there is a region where the rate is
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accelerated suddenly until the completion of the reaction. A significant enhancement in reaction rate is observed from oxygen-covered Pd surfaces under similar reaction conditions [40]. The depletion of CO in the gas phase may induce a favorable condition for the formation of oxygen
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covered surface for a higher rate.
Such variation in reaction rate is also observed at other temperatures in the temperature range
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of 175 – 250 C as shown in Figure 2(b). The reaction starts slowly and then proceeds into a steady rate. The steady rate is measured to be higher as the temperature increases. As the reaction
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approaches to completion, the rate shows a slight enhancement, which can be attributed in part to
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the decrease in CO content.
At even higher temperatures (>250 C), the reaction completes within a few tens of seconds
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and the rate is limited by the diffusion of reactant gas to the substrate [2]; at this temperature range, the measured reaction rate decreases as the reaction completes. Thus, we can assume that the diffusion rate of reactant in the gas phase does not limit the overall reaction at the temperature range of 175 – 250 C and the pressure of 0.3 Torr. Thus, the temperature and
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pressure range are selected for examining reaction kinetics on CO oxidation over the Pd/WO3
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system, such as activation energy barriers for CO oxidation.
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Figure 3. (a) The Arrhenius plot of the CO oxidation over Pd/WO3 at different stages of the reaction. (b) The calculated activation energy barrier is shown as a function of the extent of reaction.
Since the reaction rate changes over the extent of reaction, the change in the reaction rate is
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analyzed at each stage of the extent of reaction and is plotted against 1/T in Figure 3(a). In the plot, we find that the rate is higher with increasing the extent of reaction at each temperature
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until the completion of reaction. Also, at a fixed extent of reaction, the rate is higher as the
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temperature increases. At each stage of the reaction or the extent of reaction, we can determine the slope in the Arrhenius plot and it is clearly seen that the slopes are different with each other; it is observed to be high at the very early stage of reaction and decreases as the reaction approaches to a completion. It indicates that the apparent activation energy barrier is higher (lower) at the early (final) stage of the reaction; this explains the observation in Figure 2(a) that the rate is lower (higher) at the early (final) stage of the reaction. The change in the measured
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activation energy against the extent of reaction is shown in Figure 3(b). The activation energy barrier at the beginning is about 115 kJ/mol, but it rapidly approaches to a typical value of about 100 – 105 kJ/mol during the reaction (20 – 80 %). Then, it decreases to about ~ 80 kJ/mol as the
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reaction approaches to a completion. The turnover frequency at the reaction site may change as the reactant concentrations at the reaction site as well as in the gas phase change. In the beginning of reaction, a high coverage of CO on Pd surface is expected due to the high desorption temperature (~ 600 K) of CO on Pd;
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surface-bound CO may show a poisoning effect and reduce the reaction rate of CO oxidation over transition metals such as Pd [27] and Pt [5, 6, 41]. As the reaction proceeds and CO is consumed in the gas phase, the partial pressure of CO decreases. The corresponding decrease in the surface CO concentration may cause the gradual increase of reaction rate over the course of
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reaction.
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The oxidation state of Pd supported on the WO3 film may also change during the course of reaction. The measured energy barrier of 105 kJ/mol agrees well with those obtained for oxide
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supported small Pd particles [42, 43]. Higher energy barriers of 115 – 120 kJ/mol have been obtained from CO-inhibited low-temperature region over single crystal Pd surfaces–where
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reaction is limited by desorption of CO from Pd surface at < 580 K [42]. Thus, the low reaction rates with the high activation energy barriers observed at the beginning of reaction can be
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assigned for the reactivity of CO on a metallic Pd surface with high concentration of CO. As the reaction proceeds, surface oxygen population increases to form chemisorbed oxygen or a surface oxide layer (e.g., PdO) and the reaction rate is dominated by those involving those active oxygen species. As the reaction approaches to a completion, the gas phase CO concentration is reduced and the surface is more dominated by those on PdO. The activation energies for CO oxidation on
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a thin surface oxide (PdO) are suggested to be about 0.5 – 0.9 eV [25, 44]. Also, oxide supported Pd clusters can be readily oxidized into supported PdOx cluster [45] over which CO oxidation is proposed to occur via Mars-van Krevelen mechanism. Facile oxide formation can be is in part
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inferred from the similarity in CO oxidation between supported oxide clusters and the single
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crystal Pd surfaces [26, 29].
Figure 4. Cycles of CO oxidation reactions over Pd/WO3 in a fixed volume performed at PT = 0.3 Torr and Ts = 225 C. The net reaction rate increases with increasing cycles for both reduced and oxidized Pd/WO3, until a saturation rate is achieved at sufficiently high number of reaction cycles (> 8). The pictures of the Pd/WO3 film are also shown in the figure for the beginning and the end of the cycles.
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The relation between the catalytic CO oxidation activity and the initial oxidation states of Pd/WO3 is examined in Figure 4. The Pd/WO3 film is either reduced in H2 (P(H2) = 300 Torr) or oxidized in O2 (P(O2) = 300 Torr) at 300 C prior to the cycles of reactions. Then, the successive
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measurements of CO oxidation reaction rates are performed at the substrate temperature of 225 C with the mixture of CO and O2 (2:1 ratio) at P = 0.3 Torr. At the completion of CO oxidation in each reaction cycle, the reaction tube is pumped down below 10 -4 Torr while the substrate temperature is maintained to 225 C. Then, the mixture of fresh CO and O2 is introduced to the
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tube subsequently up to the same initial pressure (0.3 Torr) for the next cycle of reaction.
Figure 4 shows that the reaction rate is low for the first cycle for both cases; especially, the rate is much lower on the oxidized Pd/WO3 than on the reduced Pd/WO3; it takes about 1000 s for the completion of reaction on the reduced Pd/WO3 while it takes 4 times longer time (~ 4000 s) for
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the oxidized Pd/WO3. As the reaction cycle continues, we see that the rate becomes faster as the
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number of cycle increases until a saturation reaction rate is achieved after about 8 cycles; the reaction completes within 500 s when the reaction rate approaches to the maximum rate.
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Interestingly, the same saturation rate is achieved for both oxidized and reduced Pd/WO3 films, suggesting that the final chemical states of Pd/WO3 after the cycles of reactions would be the
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same regardless of the initial oxidation states.
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Figure 5. The change in the CO oxidation rate observed in Figure 4 is plotted with increasing number of cycles for both reduced (blue) and oxidized (red) Pd/WO3. The reaction rate approaches to a saturation value of 1.2 ×1015 CO2/s for both cases.
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The reaction rates calculated from the results in Figure 4 are plotted in Figure 5. Here, the rate is plotted against the cumulative total number of CO2 produced over cycles. At each cycle, the
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reaction rate changes over the extent of reaction. But, with increasing number of cycles, we find that the reaction rate increases for both reduced and oxidized Pd/WO3 films. The saturation rate
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is estimated to be about 1.2 × 1015 CO2/s for both cases. The observed variation in the reactivity
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strongly suggests that the surface chemical structure of the Pd/WO3 film evolves to an ‘active form’ with ‘active sites’ for CO oxidation reaction as the reaction proceeds. The active state is likely to be a partially oxidized form since both reduced and oxidized states of the Pd/WO3 film are not as active as the final ‘active state’ obtained after the long cycles of reaction in the CO and O2 mixture.
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The active form of Pd/WO3 is further annealed in vacuum at around 300 C and the reaction rate is also compared with those oxidized and reduced forms in Figure 5. The measured rate overlaps with that of the active states of Pd/WO3. The difference is that the rate is slightly
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enhanced at the end of reaction cycle as seen in Figure 2. The vacuum annealing would induce desorption of oxygen from the Pd layer which results in more metallic Pd exposed to the surface. In such a case, oxygen-covered Pd surface can be induced when CO depletes at the near end of
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the reaction for a condition for a hyperactive state for CO oxidation reaction.
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3.3 X-ray photoelectron study of the Pd/WO3 film
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Figure 6. Pd 3d core level spectra taken from Pd/WO3 at three different stages of as-synthesized, oxidized and reduced forms and those after CO oxidation.
To understand the changes in the oxidation state of Pd of our Pd/WO3 under different
preparation conditions of as-synthesized, oxidized and reduced states as well as those after CO oxidation, we show XPS spectra of Pd 3d core level taken from those samples in Figure 6. It shows that the major peak positions of Pd 3d core level for the as-synthesized and the reduced states are very similar to each other; the Pd 3d5/2 peak appears at 335.1 eV for both cases. On the
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other hand, the Pd peak for the oxidized Pd/WO3 is observed at 337 eV. Interestingly enough, the Pd core level spectral shapes of all three samples change after the CO oxidation in a way that they look very similar with each other. The major peaks for all the samples are positioned close
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to 335. 1 eV with a shoulder feature at 336.9 eV. Pd 3d5/2 peak has been observed at 335.0 – 335.2 eV for Pd/WO3 [46, 47]. Thus, the peak at 335.1 eV is attributed to thin metallic Pd films (Pd0) on WO3. Pd on WO3 prefers to be Pd2+ on WO3 [12, 48] and the binding energies of PdO are shifted by about 2 eV toward higher binding energies compared to that of metallic Pd. The Pd
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3d5/2 binding energies from PdO2 shift more to higher binding energies (up to about 340 eV) [49]. Thus, the observed peaks at 336.9 eV can be attributed to the oxidized state of Pd such as Pd2+ as in PdO. From this assignment, it is clear that the chemical states of Pd on the active form of Pd/WO3 is neither a metallic Pd, nor an oxidized PdO, but a mixture of Pd and PdO. The active
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form of mixtures (Pd and PdO) is reversibly formed during the cycles of CO oxidation reaction.
Figure 7. W 4f core level spectra taken from Pd/WO3 at three different stages of as-synthesized, oxidized and reduced forms and those after CO oxidation.
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Figure 7 shows W 4f core level spectra taken from the three samples before and after the CO oxidation. For the oxidized Pd/WO3, only a single W 4f7/2 peak at 35.8 eV is observed. This value is in the range of measured binding energies (35.3 – 35.8 eV) for W 4f7/2 peaks of WO3
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(W6+) [49-52]. As-synthesized Pd/WO3 displays the very similar spectral shape with the major peak at the same peak position as the oxidized one, suggesting that WO3 is the dominant phase in the as-synthesized one. But the W 4f7/2 peak has a low binding energy tail, suggesting the presence of additional component at lower binding energies. This is assigned to be W5+ [53].
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After reduction in H2, the W 4f7/2 peak displays a new distinct peak at even lower binding energy of 32 eV. This peak position is likely to be assigned to W2+ [32, 33, 38]. The shoulder feature at 34 eV is assigned to W4+ considering the earlier studies which assigned the W 4f7/2 peaks at 32.7 – 34.5 eV to W4+ [54-56].
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After the CO oxidation, we find again that the W 4f core level spectral shapes of all three
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samples change in a way that they look very similar with each other. The spectral shape shows a major peak at around 36.2 eV with a slope toward lower binding energies. The slope is fitted
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with a W 4f7/2 component at 35.2 eV, which can be associated with W5+ species. Thus, the fully oxidized WO3 in the Pd/WO3 system is not the most active phase. Also, the reduced state with
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mixed oxidation states of W4+ and W2+ is not active, either. Instead, the oxidized(reduced) states of WO3 in the Pd/WO3 system become reduced(oxidized) reversibly during the catalytic cycles
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of CO oxidation into the ‘active state’ of WO3 which is composed of a major component of W6+ with a contribution of W5+ state. This result shows the capability of WO3 to absorb(release) oxygen into(from) the lattice at the temperature of only 225 C during the catalytic cycles of CO oxidation reaction.
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Figure 8. O 1s core level spectra taken from Pd/WO3 at three different stages of as-synthesized, oxidized and reduced forms and those after CO oxidation. O 1s overlaps with Pd 3p3/2.
The corresponding O 1s core level spectra are explored in Figure 8. The O 1s features taken
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from the three different samples indicate that the major peak associated with WO3 appears at
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532.2 eV. This can be associated with surface OH species [57], but also the feature can overlap with the Pd 3p3/2 feature which may dominate at this position. The bulk component of O 1s from
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WO3 is not resolved and appear as a slope in the low binding energy side of the major peak for the as-synthesized and reduced cases. But, it appears as a distinct peak at 530.8 eV for the
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oxidized one. Since all the three samples are exposed to air before XPS analysis, all the features that are associated with the oxygenate species (e.g., H2O) weakly adsorbed on the surface may
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also be present at the surface and constitute the slope toward higher binding energies [58]. The O 1s contribution from PdO is not resolved in this case due to the overlap of the O 1s components from both oxides at around 530 – 531 eV. After the CO oxidation reaction, the bulk component of WO3 is relatively well distinguished from the contribution of Pd 3p3/2 as well as that of surface OH. Probably, the handling of the samples in inert N2 atmosphere after the CO oxidation has
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limited surface contamination by moisture and has left the features originating from the in-situ condition clearly resolved. Our XPS results show that the transformation of the reduced (oxidized) form of Pd/WO3 into
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the catalytically active form involves the incorporation (release) of the reactant oxygen (lattice oxygen) into the lattice (the gas phase as CO2). The oxidation states of Pd (Pd2+) and W (W6+) in the oxidized form decrease to Pd0 and W5+ simultaneously during the CO oxidation cycles. During this process, the lattice oxygen atoms in the PdO and WO3 need to be released into the
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gas phase as CO2 via CO oxidation. Here, the lattice oxygen atoms in the PdO layer can be directly consumed by the reaction with CO to form CO2. However, the oxygen in WO3 needs to be transferred to the Pd layer for the reaction with CO to form CO2 which is subsequently released into the gas phase. Similarly, the oxidation of the reduced form (Pd/WOx) with Pd0 and
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W4+ (as well as W2+) species into the active form occurs via a simultaneous increase of the
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oxidation states of both Pd and W, which requires the consumption of oxygen from the reactant gas. The oxidation of the reduced W species may occur via a direct reaction of WOx with O2 or a
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transfer of oxygen from the Pd layer into the WOx. The above two processes of transformation are reverse to each other in terms of the oxygen flow in the matrix of Pd/WO3. The oxidation of
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the reduced form is the consumption of the reactant O2 into the lattices of Pd/WOx and the reduction of the oxidized form is the release of the lattice oxygen atoms into the gas phase as
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CO2.
During the gradual change in the oxidation states of the Pd/WO3 film into the active state, the
consumption (release) of oxygen from the stoichiometric mixture of CO and O2 would cause an imbalance in the stoichiometric consumption of all reactant molecules at the end of each cycle. The amount of O2 that may be consumed (released) from (into) the gas phase from the reduced
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(oxidized) Pd/WO3 film is calculated from the size of the sample and the thickness of the Pd layer and is estimated to be about 2 x 1016. Considering the total O2 molecules involved in the reaction cycles (~ 1018), it is likely that less than 1 % of O2 is used to induce the gradual changes
stoichiometric reaction between CO and O2 at each cycle.
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in the oxidation states at each cycle. This fact makes us to assume a near completion of
The gradual changes in the oxidation states of the reduced and oxidized forms continue until the active form is achieved. The catalytic cycle of CO oxidation occurs without any further
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change in the oxidation states of Pd and W at the active state with the highest CO oxidation rates. Thus, the active form of Pd/WO3 can be considered to be the result of an equilibrium between the two processes of oxidation and reduction within the matrix of Pd/WO3. This observation can be only successfully explained under the assumption of (1) the facile exchanges of oxygen
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between as well as within the lattices of Pd and WO3 and (2) the promotional role of the oxygen
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transfer between Pd and WO3 in the catalytic CO oxidation rate over the Pd/WO3 catalyst. The promotional role of the oxygen exchange of oxygen between Pd and WO3 can be further
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evidenced from the observation of the oxidation states of Pd and W in the active form. Figure 6 reveals that Pd in the active phase consists of both metallic Pd and PdO. Also, the results in
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Figure 7 show that the tungsten oxide support layer prefers the mixed states of W5+ and W6+ state (as in WO3) in its active form over which the highest catalytic reaction rates are observed. The
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transfer of oxygen between and within the matrix of Pd and WO3 is likely to be enhanced when the catalyst is in the mixed oxidation states as observed in this study. The observation of the mixed oxidation states in its active form is consistent with the early study suggesting that the reversible formation of reduced W5+ state promotes Pd to act as an ‘oxygen scavenger’ [37]. The reduced W (W5+) may react with O2 in the gas phase or capture oxygen from PdO to become W6+.
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Then, the oxidized WO3 may lose oxygen to Pd to form the W5+ species. The facile transfer of oxygen from WO3 to Pd can promote CO oxidation by providing a labile oxygen on Pd. However, an even more direct evidence on the role of WO3 as a reversible oxygen storage in CO
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oxidation may be obtained from an in-situ observation of any changes in the chemical states under reaction condition.
4. Conclusions
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In conclusion, we have measured the CO oxidation rate over Pd/WO3 films prepared on a glass substrate at the substrate temperature of 150 – 250 °C and a pressure less than 1 Torr. Under the exposure of reactant gas mixture with the stoichiometric CO/O2 ratio of 2:1, we find that the kinetic rate follows Arrhenius behavior with activation energies predicted for CO oxidation over
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the Pd layer. In addition, the catalytic CO oxidation kinetics is strongly dependent on the
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chemical states of the Pd/WO3 catalyst. Both the oxidized and the reduced forms are not as active as the active form which has partially oxidized Pd layers supported on WO3-x. The active
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form is spontaneously obtained during the cycles of CO oxidation by a gradual change in the oxidation states of Pd and W from the oxidized and reduced states into the mixed oxidation states
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of Pd (Pd0 and Pd2+) and W (W5+ and W6+). The observation of the facile oxygen flow within the matrix during CO oxidation especially in its active form is consisted with the promotional role of
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the WO3 support as a reversible oxygen storage in the catalytic CO oxidation over the Pd layer. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
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Technology (NRF-2015R1D1A1A02062151 & NRF-2016R1D1A1B03931639) and the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, industry & Energy (No.
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20154010200820).
References
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[1] Z. Duan, G. Henkelman, ACS Catal., 4 (2014) 3435-3443. [2] F. Gao, Y. Wang, Y. Cai, D.W. Goodman, J. Phys. Chem. C, 113 (2009) 174-181. [3] F. Gao, Y. Cai, K.K. Gath, Y. Wang, M.S. Chen, Q.L. Guo, D.W. Goodman, J. Phys. Chem. C, 113 (2009) 182-192. [4] S. Salomons, R.E. Hayes, M. Votsmeier, A. Drochner, H. Vogel, S. Malmberg, J. Gieshoff, Appl. Catal. B, 70 (2007) 305-313. [5] E.M.C. Alayon, J. Singh, M. Nachtegaal, M. Harfouche, J.A. van Bokhoven, J. Catal., 263 (2009) 228-238. [6] J. Singh, E. Alayon, M. Tromp, O.V. Safonova, P. Glatzel, M. Nachtegaal, R. Frahm, J.A. van Bokhoven, Ang. Chem., 47 (2008) 9260-9264. [7] R. Imbihl, G. Ertl, Chem. Rev., 95 (1995) 697–733. [8] M. Valden, X. Lai, D.W. Goodman, science, 281 (1998) 1647-1650. [9] H. Gandhi, G. Graham, R.W. McCabe, J. Catal., 216 (2003) 433-442. [10] R.M. Heck, R.J. Farrauto, Appl. Catal. A, 221 (2001) 443-457. [11] Y. Zhou, Z. Wang, C. Liu, Catal. Sci. Technol., 5 (2015) 69-81. [12] K. Okumura, M. Niwa, Catal. Surv. Jpn., 5 (2002) 121-126. [13] S.N. Pavlova, V.A. Sadykov, V.A. Razdobarov, E.A. Paukshtis, J. Catal., 161 (1996) 507516. [14] A. Satsuma, K. Osaki, M. Yanagihara, J. Ohyama, K. Shimizu, Appl. Catal. B, 132–133 (2013) 511-518. [15] E.M. Slavinskaya, Y.T. Kardash, O.A. Stonkus, R.V. Gulyaev, I.N. Lapin, V.A. Svetlichnyi, A.I. Boronin, Catal. Sci. Technol., 6 (2016) 6650-6666. [16] J. Wang, H. Chen, Z. Hu, M. Yao, Y. Li, Catal. Rev., 57 (2015) 79-144. [17] K.M. Adams, H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev., 22 (1983) 207-212. [18] Y. Sakai, A. Shimanaka, M. Shioi, S. Kato, S. Satokawa, T. Kojima, A. Yamasaki, Catal. Today, 241, Part A (2015) 2-7. [19] C. Bigey, G. Maire, J. Catal., 196 (2000) 224-240. [20] M.A. Kulagina, E.Y. Gerasimov, T.Y. Kardash, P.A. Simonov, A.V. Romanenko, Catal. Today, 246 (2015) 72-80. [21] Z. Lou, Q. Gu, Y. Liao, S. Yu, C. Xue, Appl. Catal. B, 184 (2016) 258-263. [22] Y.-A. Lee, S.S. Kalanur, G. Shim, J. Park, H. Seo, Sens. Actuators, B, 238 (2017) 111-119. [23] N.-H. Kim, S.-J. Choi, D.-J. Yang, J. Bae, J. Park, I.-D. Kim, Sens. Actuators, B, 193 (2014) 574-581.
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
CR IP T
[24] P. Van Tong, N.D. Hoa, N. Van Duy, D.T.T. Le, N. Van Hieu, Sens. Actuators, B, 223 (2016) 453-460. [25] J.T. Hirvi, T.-J.J. Kinnunen, M. Suvanto, T.A. Pakkanen, J.K. Nørskov, J. Chem. Phys., 133 (2010) 084704. [26] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci., 552 (2004) 229-242. [27] S.M. McClure, D.W. Goodman, Chem. Phys. Lett., 469 (2009) 1-13. [28] G. Zheng, E.I. Altman, J. Phys. Chem. B, 106 (2002) 1048–1057. [29] S. Blomberg, M.J. Hoffmann, J. Gustafson, N.M. Martin, V.R. Fernandes, A. Borg, Z. Liu, R. Chang, S. Matera, K. Reuter, E. Lundgren, Phys. Rev. Lett., 110 (2013) 117601. [30] A.S. Ivanova, E.M. Slavinskaya, R.V. Gulyaev, V.I. Zaikovskii, О.А. Stonkus, I.G. Danilova, L.M. Plyasova, I.A. Polukhina, A.I. Boronin, Appl. Catal. B, 97 (2010) 57-71. [31] T. Schalow, M. Laurin, B. Brandt, S. Schauermann, S. Guimond, H. Kuhlenbeck, D.E. Starr, S.K. Shaikhutdinov, J. Libuda, H.J. Freund, Ang. Chem., 44 (2005) 7601-7605. [32] I. Jirka, J. Plšek, F. Šutara, V. Matolín, V. Cháb, K.C. Prince, J. Phys. Chem. B, 110 (2006) 23837-23844. [33] J. Plšek, I. Jirka, V. Nikolajenko, Z. Knor, Surf. Sci., 600 (2006) 3943-3949. [34] A. Hadi, I.I. Yaacob, Catal. Today, 96 (2004) 165-170. [35] S.-H. Oh, G.B. Hoflund, J. Phys. Chem. A, 110 (2006) 7609-7613. [36] L. Meng, A.-P. Jia, J.-Q. Lu, L.-F. Luo, W.-X. Huang, M.-F. Luo, J. Phys. Chem. C, 115 (2011) 19789-19796. [37] A.M. Sica, C.E. Gigola, Appl. Catal. A, 239 (2003) 121-139. [38] K. Mašek, S. Nemšák, M. Mravcáková, P. Blumentrit, T. Skála, M. Škoda, V. Matolín, J. Phys. Conf. Ser., 100 (2008) 012008. [39] L. Liu, F. Zhou, L. Wang, X. Qi, F. Shi, Y. Deng, J. Catal., 274 (2010) 1-10. [40] M.S. Chen, Y. Cai, Z. Yan, K.K. Gath, S. Axnanda, D.W. Goodman, Surf. Sci., 601 (2007) 5326-5331. [41] A.D. Allian, K. Takanabe, K.L. Fujdala, X. Hao, T.J. Truex, J. Cai, C. Buda, M. Neurock, E. Iglesia, J. Am. Chem. Soc., 133 (2011) 4498-4517. [42] D.W. Goodman, Chem. Rev., 95 (1995) 523-536. [43] L. Kieken, M. Boudart, Catal. Lett., 17 (1993) 1-10. [44] J. Rogal, K. Reuter, M. Scheffler, Phys. Rev. Lett., 98 (2007) 046101. [45] B. Huber, P. Koskinen, H. Hakkinen, M. Moseler, Nat. Mater., 5 (2006) 44-47. [46] S. Kim, S. Park, S. Park, C. Lee, Sens. Actuators, B, 209 (2015) 180-185. [47] M. Ranjbar, N. Tahmasebi Garavand, S.M. Mahdavi, A. Iraji zad, Sol. Energy Mater. Sol. Cells, 94 (2010) 201-206. [48] K. Okumura, T. Kobayashi, H. Tanaka, M. Niwa, Appl. Catal. B, 44 (2003) 325-331. [49] A. Boudiba, P. Roussel, C. Zhang, M.-G. Olivier, R. Snyders, M. Debliquy, Sens. Actuators, B, 187 (2013) 84-93. [50] P. Charton, L. Gengembre, P. Armand, J. Solid State Chem., 168 (2002) 175-183. [51] J.N. Fiedor, A. Proctor, M. Houalla, D.M. Hercules, Surf. Inter. Anal., 23 (1995) 204-212. [52] A. Boudiba, C. Zhang, P. Umek, C. Bittencourt, R. Snyders, M.-G. Olivier, M. Debliquy, Int. J. Hydrogen Energy, 38 (2013) 2565-2577. [53] K. Mašek, J. Libra, T. Skála, M. Cabala, V. Matolín, V. Cháb, K.C. Prince, Surf. Sci., 600 (2006) 1624-1627. [54] W. Grünert, E.S. Shpiro, R. Feldhaus, K. Anders, G.V. Antoshin, K.M. Minachev, J. Catal., 107 (1987) 522-534.
ACCEPTED MANUSCRIPT
AC
CE
PT
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M
AN US
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[55] P. Biloen, G.T. Pott, J. Catal., 30 (1973) 169-174. [56] C. Bigey, L. Hilaire, G. Maire, J. Catal., 184 (1999) 406-420. [57] J. Li, Y. Liu, Z. Zhu, G. Zhang, T. Zou, Z. Zou, S. Zhang, D. Zeng, C. Xie, Sci. Rep., 3 (2013) 2409. [58] J.-C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys., 2 (2000) 1319-1324.
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Graphical Abstract